Polarimetric Characteristics of Dust Particles as Observed in 13 Comets: Comparisons with Asteroids

Abstract

Polarimetric observations of 13 comets are presented, which were conducted mainly with the narrow-band International Halley Watch (IHW) filters. The analysis shows that the dusty comets in the sample form a separate group. Within measurement errors these comets display the same polarization-phase angle dependence. The wavelength dependence of polarization is not noticable at small phase angles α ≤ α0 (the inversion angle). For α > α0 the polarization increases with wavelength, with a larger increase at larger phase angles. In our view the negative polarization is produced by fluffy particles, large as compared to the wavelength, and therefore does not depend on wavelength, whereas at larger phase angles smaller particles contribute increasingly more to the polarization and cause a wavelength dependence. The slope of the positive branch of the polarization-phase curve increases at phase angles exceeding 45°; so instead of a single slope h, as observed in asteroids, two slopes h1 and h2 >h1 must be used to describe the polarization at phase angles smaller or larger than 45°, respectively. This again can be explained by the contribution of small particles, which, contrary to the situation in asteroids, act as independent scatterers and start to contribute significantly to the polarization at larger phase angles. The comets with faint continuum, i.e., the gas-rich comets, exhibit a different polarization. Comparison with photometric measurements indicates that a large part of the difference can be ascribed to a contamination of even the narrow-band IHW filters by molecular emissions but a real difference seems to be present. Possibly, small particles are underabundant in gas-rich comets.